1. Field of the Invention
The present invention relates to a thread trimming device for a sewing machine.
2. Description and Problems of the Related Art
There is known a conventional thread trimming device for a sewing machine which cuts needle and bobbin threads by reciprocating a movable knife coupled to a thread trimmer which has a projection engaging a cam surface of a thread trimmer cam fixed to a hook driving shaft provided under the bed of the sewing machine, the hook driving shaft rotating in synchronism with the arm shaft of the sewing machine. In such a thread trimming device, a needle is stopped at the position adjacent to the lower dead point thereof (point A in FIG. 9), the, a rotary solenoid provided adjacent to the thread trimmer cam is operated forward (point B in FIG. 9) to let the cam roller of the trimmer lever engage the cam groove of the thread trimmer cam, so as to reciprocate the movable knife by way of a link mechanism coupled to the thread trimmer lever which moves along the above mentioned cam groove for cutting the needle and bobbin threads in cooperation with a fixed knife. After the completion of cutting the threads by the movable and fixed knives, the rotary solenoid is operated backward to separate the cam roller of the thread trimmer lever from the cam groove of the thread trimmer cam so as to complete the operation of trimmer the threads. In such a conventional thread trimming device for a sewing machine, the adjustment of tinting of completing the thread trimming is a time-consuming and troublesome work since the arm shaft is turned little after the time (point K in FIG. 9) when the thread trimming is completed (that is, when the threads are cut, the movable knife is returned to its rest position by way of the thread trimmer cam and moreover the rotary solenoid is deenergized) until the time when the needle is stopped at the upper stop position (point H in FIG. 9).
When the thread trimming is adjusted too early, there occurs the troubles of;
(1) mistrimming of threads; or PA1 (2) the interruption of sewing since the needle thread pulls out of the throat when sewing is resumed as the throat thread end remains short at the needle point due to the rising of the thread take-up lever after the completion of the thread trimming. While the thread trimming is adjusted too late, there occurs the troubles in handling cloths etc. when sewing is completed since the needle is stopped (point H in FIG. 9) after the thread take-up lever passes the upper dead point thereof corresponding to the correct upper stop position of the needle and consequently the needle is stopped at the position too close to the needle plate.
Moreover, the rotary solenoid has to be energized during the time between points B and K in FIG. 9 so as to continue the operation of the rotary solenoid for a long time, resulting in the waste of energy. At point B an operator operates a pedal to supply a thread cutting instruction to the sewing machine, which starts rotation at a low speed and the rotary solenoid starts a forward operation upon reception of the instruction.